Also, prosecutors commonly encourage people to plead guilty to a lesser charge (even when the person insists they're innocent), by talking up their case as though there's no chance the person will "get off" if they go to trial. That tactic on the part of the prosecutor can be absolutely terrifying.
if you plead guilty to child sexual abuse you better expect that normal mentally healthy people are not going to give you access to their children. Period. You aren't even allowed within X distance from schools or places children congregate. I'd be extremely surprised if he is even legally allowed to live in the same home with children. So that right there is a HUGE red flag. We have no evidence to assume that OPs SIL lied when she told him she didn't know. In fact it's much more likely if she did know she would have said so. All she did was make herself look even worse for moving in with him after being told of his conviction.